Abstract
Ph.D.
One of the solutions to the high cost of solar modules is the development of thin film
solar cell technologies, which enable material saving, few processing steps, good
stability in outdoor testing, high conversion efficiency and flexibility for large area
coatings. Polycrystalline CuInSe2 (CIS) thin films and related quaternary and
pentenary compounds such as Cu(In,Ga)Se2 (CIGS) and Cu(In,Ga)(Se,S)2 (CIGSS)
are the most promising thin film candidates to fulfil the requirements of economically
viable solar modules. Presently CIS, CIGS and CIGSS thin film solar cells are
prepared mostly by two – stage deposition processes, where Cu-In-Ga alloys are
deposited, followed by selenization and/or sulfurization using H2Se/Ar and/or H2S/Ar
gases, Se and/or S vapours. Key problems related to this approach are (1) the widely
reported compositional change and loss of material during the annealing and
selenization stages, and (2) the formation of a graded film structure with most of the
Ga residing at the back of the film, due to the difference in the reaction rates between
the binary selenides.
The present study aims to develop CIGS quaternary and CIGSS pentenary thin film
absorbers which are substantially homogeneous and single phase. In order to
achieve this aim different deposition processes were developed. This included
thermal evaporation of pulverized compound materials from a single crucible with and
without subsequent reaction of the precursors in Se vapour or H2Se/Ar atmosphere.
Alternatively, controlled partial selenization/sulfurization of the Cu-In-Ga magnetron
sputtered precursor films under controlled conditions of reaction time, temperature
and gas phase concentration were applied to produce CIGSS films. The latter
approach allowed homogeneous incorporation of Ga and S species into CIS
compound material, and with that a corresponding increase of band gap of the
material in the active region of the solar cell.
CIGS quaternary and CIGSS pentenary based solar cells were completed by
depositing a CdS buffer layer of around 50 nm thickness, high resistivity ZnO and low
resistivity Al – doped ZnO with thicknesses of about 50 nm and 0.5 μm respectively.
I-V measurements on fabricated solar cells, under standard A.M. 1.5 conditions,
demonstrated good solar cell device quality with efficiencies of about 10 % and 15%
respectively.